You would have to pay to sit these exams in a school that offers (I)GCSEs in your area. You would have to contact the school and ask if you could sit your exam there (Usually, the earlier you ask the cheaper it is and the more likelier they will allow you). If they offer Edexcel IGCSEs, then the Oxford Open Learning is an option for you
(Sorry if you already know this). For (I)GCSEs, there are different exam boards you can enter in for. You need to contact your school of choice and ask them what exam boards they pursue. You need to try to do exams with no controlled assessments as these might take up extra time and schools often don't allow private candidates like yourself to pursue this -IGCSEs are usually better at this than GCSEs (However, a school allowed me for my GCSE Chinese)
Exam boards I would recommend
For Maths: Edexcel/AQA GCSE Maths - has no controlled assessments (TBH, I don't know any board which has coursework for Maths
For Economics: You could do Edexcel iGCSE Economics as it doesn't have any controlled assessments
In terms of study, you should first contact a school and decide what exam board you're doing (e.g. Edexcel, AQA, OCR, Cambridge, WJEC, CCEA). It is a law that you must do English, Maths and Science with a least a C grade in each
Then, you need to see where you're in terms of knowledge so I suggest picking up a past paper for each subject (I can show how to get these later) and see what happens. After that, you can come up with a plan as you would know how much you need to work to get your target grade. Also, look into fast track courses.
P.S. iGCSE means International GCSE
In most GCSEs there are 2 tiers - Foundation (ranges from G to C) and Higher (ranges from C to A*)
Sorry for typing loads but I'm just trying to help